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Let $X$ be a $T_1$ topological space. Show that $X$ is connected and locally connected if and only if for each open cover $\{U_s\}_{s\in S}$ and any $x, y \in X$, there exist $s_1,. . . , s_n \in S$ and $V_1,. . . , V_n$ connected open in $X$ such that $x \in V_1$, $y \in V_n$, $V_i \subset U_{s_i}$ for all $i \in \{1,. . . , n\}$ and $V_i \cap V_j \neq \emptyset$ if, and only if, $| i - j | \leq 1$.

My idea: Let's suppose for each open cover $\{U_s\}_{\{s\in S\}}$ and any $x, y \in X$, there exist $s_1,. . . , s_n \in S$ and $V_1,. . . , V_n$ connected open in $X$ such that $x \in V_1$, $y \in V_n$, $V_i \subset U_{s_i}$ for all $i \in \{1,. . . , n\}$ and $V_i \cap V_j \neq \emptyset$ if, and only if, $| i - j | \leq 1$ and let's see that $ X $ is connected and locally connected

  1. Connected. Suppose $X$ is disconnected; that is, $X = U_1 \cap U_2$, where $U_1 \cap U_2 = \emptyset$, $U_1$, $U_2$ open not empty. Notice that $\mathcal{U}= \{U_1, U_2\}$ is an open cover of $X$. Lets $x \in U_1$ and $y \in U_2$, by hypothesis there are $s_1,s_2 \in \{1,2\}$ and $V_1, V_2$ connected open in $X$ such that $x \in V_1$, $y \in V_2$, $V_i \subset U_{s_i}$ for all $i \in \{1, 2\}$, since U and V are disjoint, it contradicts that $V_1 \cap V_2 \neq \emptyset$. therefore $ X $ is connected.
  2. locally connected. Let $x \in X$, and $U$ an open of $ X $ such that $x \in U \subset X$, let $V$ be a closed neighborhood of $x$ in $X$ such that $V \subset U$ consider cover $\mathcal=\{U, X \setminus V\}$. let $x \in U$, $y \in X \setminus U$ by hypothesis there are $V_1$ open connected such that $x \in V_1 \subset U$. I am not sure if this last argument is correct.

The other implication I'm stuck, some help please

Paul Frost
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user08
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1 Answers1

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Your question is a duplicate of Characterization of Connected Locally Connected Sets where you can find all proofs. However, you wanted your ideas to be checked:

The proof of 1. is correct.

The proof of 2. is not correct because you cannot be sure that there exists a closed neigborhood $V$ of $x$ with $V \subset U$. But you can take $X \setminus \{x\}$ instead of your $X \setminus V$, and you should not require $y \in X \setminus \{x\}$ because the latter set could be empty. Take any $y \in X$, if you want $y = x$. Moreover, $V_1 \subset U$ needs an argument, from the definition we only know that $V_1 \subset U$ or $V_1 \subset X \setminus \{x\}$. Now note that the second inclusion is impossible.

Paul Frost
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